Functional characterization of spatial neglect in Parkinson's disease
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Parkinson’s disease (PD) is associated with perceptual as well as motor disturbances. Unlike those with right-side onset (RPD), individuals whose motor symptoms begin on the left side of the body (LPD) may experience mild neglect of left hemispace. The functional cause of these perceptual abnormalities in LPD is unknown; possibilities include altered eye movements, changes in perceptual representation of left versus right hemispace, and attentional abnormalities. Three studies explored these possibilities in non-demented individuals with mild-moderate idiopathic PD (24-36 per experiment) and matched normal control adults (NC; 14-24 per experiment). Study 1 consisted of two psychophysical line-bisection experiments that used brief stimulus presentations to preclude exploratory eye movements. In both, participants judged whether a systematically adjusted hatchmark was left or rightward of a horizontal line’s midpoint. In the second, eye tracking was used to ensure that participants fixated during stimulus onset. The first experiment revealed neglect-like performance in LPD, but the second did not, suggesting that neglect may arise from biased eye gaze but not from abnormal saccades. Study 2 evaluated two hypotheses to explain spatial bias in LPD; one an active hypothesis in the literature that the representation of left hemispace is compressed, and the other a novel hypothesis that left hemispace is less salient than the right in LPD. Both hypotheses were assessed psychophysically, using spatial frequency and contrast discrimination paradigms, and neither was supported. Study 3 investigated whether endogenous visuospatial attentional abnormalities exist in PD, and particularly whether LPD show weaker attentional abilities in the left hemifield. A multiple-object tracking paradigm was used along with eye tracking to ensure that participants used only endogenous (not exogenous) attention. PD showed dilated temporal resolution of attention, and were less accurate in tracking multiple objects at once. LPD did not differ from RPD for either of these metrics. Overall the results suggest that there is no intrinsic difference in the representation of space in LPD relative to RPD or NC, but that visuospatial attention is altered in PD generally. The latter finding is particularly important for considering spatial navigation in individuals with PD even in early stages of the disease.